Technical Field
The present invention relates to magnetic tunnel junction devices and, more particularly, to magnetic random access memory cells using double magnetic tunnel junctions.
Description of the Related Art
Magnetoresistive random access memory (MRAM) cells may be formed using magnetic tunnel junction structures. In such a device, a “fixed” magnetic layer is separated from a “free” magnetic layer by a thin insulating barrier. When a voltage is applied across the device, electrons tunnel through the insulating barrier by quantum effects, creating a current. The orientation of the magnetization of the free layer relative to the fixed layer determines how likely an electron is to tunnel across the barrier, such that the magnetization of the free layer determines an effective resistance of the device that can be measured.
The magnetization of the free layer may be set using, e.g., spin-transfer torque. By applying a spin-polarized current to the free magnetic layer, angular momentum is transferred to the free layer and the orientation of its magnetization can be changed. In this manner, a bit of information may be stored in the MRAM cell and subsequently read out by applying a current and determining the resistance.
However, in conventional magnetic tunnel junction devices, where a single tunnel barrier is used, the switching current used to set a state of the device can be quite high. This problem may be addressed by using double magnetic tunnel junction devices, where the free layer is sandwiched between two insulating layers, with fixed layers above and below. In such a device, the spin current from each side of the free layer may add, thereby lowering the switching current needed.
While such a structure effectively lowers the switching current, the thickness of the stack is nearly doubled relative to a single magnetic tunnel junction device. Thick device stacks take longer to grow and etch during fabrication, thereby increasing the cost and decreasing the efficiency of fabrication.